Optical pickup devices used in conventional recording and/or reproducing apparatuses in which an optical disc is used as a recording medium, includes a semiconductor laser as a light source for emitting a light beam, a collimator lens which is disposed between the semiconductor laser and the optical disc and whose optical axis is aligned with optical axes of the semiconductor laser and the optical disc, a diffraction grating, a polarizing beam splitter and an objective lens. The optical pickup device further includes a photodetector for receiving a light beam from the optical disc, and a focusing lens which is disposed between the photodetector and the polarizing beam splitter and whose optical axis is aligned with optical axes of the photodetector and the polarizing beam splitter.
In such conventional optical pickup devices, the light beam is emitted from the semiconductor laser and transmitted through the polarizing beam splitter and an objective lens so as to be radiated and focused onto an information signal storage layer of the optical disc. The light beam reflected from the optical disc is received by the photodetector.
The optical disc used as a recording medium for a recording and/or reproducing apparatus in which the afore-mentioned optical pick-up is incorporated and which is adapted to record an information signal thereon and/or reproduce the signal therefrom, generally includes a substrate made of a transparent synthetic resin material such as polycarbonate or polymethyl-methacrylate, an information signal layer formed on a main surface of the substrate, and a protecting layer formed over the information signal layer to protect the layer against chemical erosion or mechanical damage.
Such an optical disc has been produced by using various shaping methods such as a thermal pressure-molding method or a thermal casting method. When subjected to the shaping method, the optical disc is likely to suffer from undesired warpage or corrugation due to a thermal distortion generated therein. Consequently, a dislocation of the information signal storage layer occurs in the direction perpendicular to the main surface of the disc. Further, the circularity of tracks formed on the information signal layer is adversely affected. When the optical disc is rotated to reproduce the information signal stored on the information signal layer, the surface of the information signal layer and the tracks thereon suffer from undesirable oscillation and deflection.
Accordingly, in order to achieve an accurate read-out or reproduction of the information signal stored on a signal recording region of the optical disc without being adversely affected by the oscillation of the optical disc surface and the deflection of the tracks, the optical pickup device is provided with a focusing control function for conducting an adequate focusing adjustment of the objective lens in response to a degree of the eventual oscillation of the surface of the optical disc, and a tracking control function for performing an adequate tracking adjustment of the objective lens so as to follow the oscillation of a signal track formed on the signal recording region of the optical disc.
Meanwhile, there has been a demand for higher density information storage, so that proposals have been made to provide a multi-layer optical disc having a multiple of information signal layers arranged in an overlapped relation to each other. The multi-layer optical disc conventionally known is of a two-layer type having two overlapped information signal layers.
The two-layer type optical disc 5 includes, as shown in FIG. 1, a disc substrate 5C made of a transparent synthetic resin material such as polycarbonate (PC) or polymethyl-methacrylate (PMMA), a first information signal storage layer 5A formed on a main surface of the disc substrate 5C, a spacer layer 5D made of a transparent resin material and disposed over the first information signal storage layer, a second information signal storage layer 5B disposed over the first information signal storage layer 5A through the spacer layer 5D, and a protecting layer 5E formed over the second information signal storage layer 5B to protect the second information signal storage layer against mechanical damage and chemical erosion.
In such a two-layer type optical disc, the laser beam emitted from the semiconductor laser is transmitted through the first information signal storage layer 5A and radiated to the second information signal storage layer 5B when the optical pickup device performs a read-out of the information signal from the second information signal storage layer 5B disposed over the first information signal storage layer 5A.
In the optical pickup device used here, the tracking control is carried out by using a 3-spot method in which three light beams are radiated to the optical disc to obtain a tracking control signal. When the information signal stored on the individual information signal storage layers of the multi-layer optical disc is read by means of the optical pickup device which employs the above-mentioned 3-spot method to conduct a tracking control, the following problems are caused. As shown in FIG. 2, a photo-detector used in such an optical pick-up device in which a tracking control is performed by using the 3-spot method, includes a main beam detector 33A for detecting a main beam which is a centrally radiated beam of three separated light beams, and first and second side-beam detectors 33B and 33C disposed on opposite sides of the main beam detector 33A, arranged so as to sandwich the main beam detector 33A therebetween, for detecting side beams radiated on opposite sides of the main beam.
When the information signal stored on the first information signal storage layer 5A of the two-layer type optical disc 5 is read out, the main beam 34A and the first and second side beams 34B and 34C are radiated onto the first information signal storage layer 5A. In the course of the reading-out of the information signal stored on the first information signal storage layer 5A, these light beams are transmitted through the first information signal storage layer 5A and also radiated onto the second information signal storage layer 5B whose information signal is not intended to be read out.
In consequence, the photo-detector of the optical pick-up device receives not only the main beam 34A reflected from the focused first information signal storage layer 5A but also a blooming light component 35A of the main beam, namely a stray light component, reflected from the unfocused information signal storage layer SB. That is, in the optical pick-up device, the main beam 34A and the stray light component 35A having an expanded outer perimeter are radiated together onto the main beam detector 33A in a concentric relation to each other.
Accordingly, in such an optical pick-up device, a part of the stray light component 35A of the main beam radiated on the main beam detector 33A is overlappingly radiated over the side beam detectors 33B and 33C. Thus, when the part of the stray light component 35A of the main beam is radiated over the side-beam detectors 33B and 33C and overlapped with the respective side beams, there occurs direct-current (DC) fluctuation in a tracking error signal, which occasionally makes it impossible to carry out an accurate tracking control.
Consequently, in the optical pick-up device, such an advantage of the 3-spot method that the tracking error signal is not adversely affected by a skew of the disc and a deflection of a view field of the objective lens and therefore does not suffer from D.C. fluctuation, cannot be maintained. Further, when the radiation of a read light beam focused through the objective lens is switched between the information signal storage layers of the two-layer type optical disc, a form of the stray light component 35A of the main beam radiated on the main beam detector 33A is changed.
Accordingly, a ratio of a light quantity of the stray light component 35A of the main beam radiated over the respective side beam detectors 33B and 33C to those of the side beams is also changed, so that the tracking error signal suffers from D.C. offset, thereby causing such a problem that it is no longer possible to perform the accurate tracking control.
Meanwhile, an intensity of the stray light component of each side beam is considerably that of the stray light component of the main beam. In consequence, the influence of the stray light components of the side beams on the main beam detector is negligible and therefore causes no significant problem.